The use of semiconductor technology has, over the last few decades, revolutionized the use of electrical and electronic goods. In particular, the increased use of semiconductor technology has resulted from a widespread, unappeasable need by business (as well as individuals) for better, smaller, faster and more reliable electronic goods.
The semiconductor manufacturers have therefore needed to make commensurate improvements in product performance, as well as in the speed, quality and reliability of the semiconductor manufacturing process. Clearly, in the mass-manufacture of semiconductors, the manufacturer needs to minimize the number of faulty semiconductors that are manufactured. Furthermore, the manufacturer clearly needs to recognize, as early as possible in the manufacturing process, when faulty semiconductors are being manufactured, so that the manufacturing process can be checked and, if appropriate, corrected.
One particular process, in the semiconductor manufacturing process cycle, which has evolved as being critical in saving time and cost in the mass manufacture of semiconductors, is the semiconductor wafer inspection process. Various semiconductor wafer inspection processes have evolved for different stages during the semiconductor wafer manufacturing process. By continuously inspecting semiconductor wafers throughout the manufacturing process, often using optical inspection techniques, flawed wafers may be removed and, if appropriate, the manufacturing process corrected at any of the various stages. This is preferable to completing the whole wafer manufacturing process, only to find that a defect exists in a final inspection or by failure during use.
Optical sensing is the process of converting optical signals (photons) into electrical signals (electrons) and subsequently measuring the optical signal. In most applications, where the optical signals are large, or the temporal frequencies are low, such conversion is performed using solid state devices known as Photodiodes. Photodiodes are inexpensive and simple to use. They have a high dynamic range, and can be very fast when the amount of light intensity is sufficiently large.
For signals where light intensity is low, photodiodes cannot operate at high speed, due to their relatively high noise level of the diode, and the small currents generated by the low light energy signals. Even though photodiodes have excellent dynamic range, their output is proportional to the optical signal, so in practice their useful dynamic range is quickly limited by subsequent electronics.
Among the more popular photosensitive devices in use today, are phototubes, used particularly in less sensitive applications such as absorption spectrometers. Phototubes consist of a single photocathode and a single anode to convert light energy into electrical energy. However, for the vast majority of photosensitive applications, phototubes do not have the internal amplification required to provide acceptable sensitivity and performance.
Hence, photomultiplier tubes (PMTS) have been developed, particularly for use when the optical signals are of low or very low light intensity and/or when the required detection frequencies are high. PMTs have a reputation of being versatile devices that provide extremely high sensitivity, low noise and an ultra-fast response.
The PMT device has therefore provided particular benefits when used for light detection over various wavelengths with minimal noise, typically limited only by the impending statistic noise (often termed ‘shot noise’). As such, the PMT device is used for detecting light reflected and scattered off an investigated substance, in order to detect defects and other desired information about the substance.
In addition, PMTs may be used in various techniques, such as wafer inspection, printed circuit board (PCB) inspection, flat panel inspection, layers height and properties inspection, fluorescence spectrophotometry, Bio/Chemiluminescence's, liquid scintillation counting, high-energy physics and astronomy, photon counting and others.
A typical PMT configuration 100 is shown in FIG. 1. The PMT consists of a photoemissive cathode (photocathode) 115 followed by focusing electrodes (termed dynodes) 125 functioning as a photoelectron multiplier and a photoelectron collector (anode) 135 in a vacuum (or gas-filled) phototube 110. The photocathode 115 is capable of emitting a stream of photoelectrons when exposed to light. The dynode arrangement 125 provides for successive steps in amplification of the original photoelectron signal from the photocathode 115. The resulting signal produced at the anode 135 is directly proportional to the amount of illumination that entered the photocathode 115.
When light or a photon of light 105 of sufficient energy strikes the photocathode 115, the photocathode emits photoelectrons 120 into the vacuum due to the photoelectric effect. The photocathode material is usually a mixture of alkali metals, which make the PMT sensitive to photons throughout the visible region of the electromagnetic spectrum. The photocathode 115 is typically configured to be at a high negative voltage, typically −500 to −1500 volts.
The emitted photoelectrons 120 are then accelerated towards a series of additional electrodes (called dynodes) by a focused electric field 130 (typically configured by a supply voltage with a voltage divider resistor chain to provide a series of electrode voltages). When the photoelectrons strike each dynode 125 the photoelectrons dislodge additional photoelectrons (termed secondary photoelectrons), thus amplifying the signal by the process of secondary emission. These secondary photoelectrons then cascade towards the next dynode where they are again amplified. This cascading effect typically creates between 102 and 107 secondary photoelectrons for each photoelectron that is emitted from the photocathode. The amplification depends on the number of dynodes 125 and the focused electric field 130.
At the end of the dynode chain, an anode 135 at ground potential collects the multiplied secondary photoelectrons as an output signal. At this point, the output signal 140 is large enough to be easily measured using conventional electronics, such as a transimpedance amplifier, followed by an analog-to-digital converter.
Due to the secondary emission multiplication process, PMTs provide extremely high sensitivity and exceptionally low noise among the photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. The PMT also features fast time response, low noise and a choice of large photosensitive areas.
The gain at each dynode 125 is a function of the energy of the incoming secondary photoelectron, which is proportional to the electrical potential between that dynode and the previous stage. The total gain of the tube is the product of the gains from all the dynodes. Typically, and as shown in FIG. 1, connecting a string of voltage-divider resistors between the cathode, all the dynodes, and ground generates the bias voltages for the dynodes. Typically, the resistance and therefore the voltage between all of the dynodes 125 and between the last dynode and anode 135 is the same. A large negative voltage 118 is then applied to the cathode, and the potential is divided up evenly across the dynodes by the voltage-divider resistor chain of the focused electric field 130.
This conventional biasing scheme is useful for operating the photomultiplier tube at a single programmable gain. Altering the applied cathode voltage changes the gain. However, the large voltages involved make it difficult to change the gain quickly, due to parasitic capacitances and the large resistor values needed to limit power dissipation in the bias string. The conventional usage is to decide on a tube gain in advance, set the appropriate cathode voltage and then operate the tube at that voltage throughout the measurement operation.
Hence, the use of known photomultiplier tubes in an optical inspection arrangement for wafers and semiconductors has a number of significant disadvantages, not least the limited dynamic range associated with the signal amplification process and fixed gain associated with the input to output signal.
Thus, there exists a need in the field of the present invention to provide an improved method and apparatus for wafer inspection, particularly a photodetection process using photomultiplier tubes, wherein the abovementioned disadvantage may be alleviated.